The invention relates to parts based on carbon/carbon (C/C) composite material, in particular friction parts such as airplane brake disks. Nevertheless, the invention is not limited to this application and it also applies to other types of C/C composite material parts, in particular those for which improved mechanical properties are desired.
Airplane brake disks made of C/C composite material are in widespread use. The fabrication of such disks conventionally includes a step of making a fiber preform out of carbon fibers in a shape that is close to the shape of the disk that is to be fabricated, the preform serving to constitute the fiber reinforcement of the composite material, and then densifying the preform with a carbon matrix.
One well-known method of making a fiber preform out of carbon fibers comprises superposing fiber plies of carbon-precursor fibers, e.g. of pre-oxidized polyacrylonitrile (PAN), bonding the plies together, e.g. by needling, and performing carbonization heat treatment to transform the precursor into carbon. Reference may be made to document U.S. Pat. No. 5,792,715, amongst others.
The preform may be densified with a carbon matrix by performing chemical vapor infiltration (CVI). Preforms are placed in an enclosure into which a gas is admitted that contains one or more precursors of carbon, e.g. methane and/or propane. The temperature and pressure in the enclosure are controlled so as to enable the gas to diffuse within the preforms and form therein a solid deposit of pyrolytic carbon (PyC) by the precursor(s) decomposing. A method of densifying a plurality of annular preforms for brake disks placed in stacks is described in document U.S. Pat. No. 5,904,957, amongst others.
Densification with a carbon matrix may also be performed using a liquid technique, i.e. by impregnating the preform with a carbon precursor, typically a resin, and then pyrolyzing the precursor, it being usual to perform a plurality of impregnation and pyrolysis cycles.
A densification method is also known that may be said to rely on “calefaction” and in which a disk preform for densifying is immersed in a bath of carbon precursor, e.g. toluene, and is heated, e.g. by inductive coupling, so that the precursor vaporises in contact with the preform and diffuses within it so as to form a PyC deposit by decomposition. Such a method is described in document U.S. Pat. No. 5,389,152, amongst others.
Amongst the various properties looked for in brake disks based on C/C composite material, low wear is highly desirable.
In order to improve wear resistance, many proposals have been made to introduce ceramic grains in the C/C composite material.
Thus, document U.S. Pat. No. 6,376,431 describes impregnating a carbon fiber preform with a sol-gel type solution containing a precursor for silica (SiO2) that, after heat treatment and reacting chemically with the carbon, leaves grains of silicon carbide (SiC) distributed within the preform, which grains represent no more than 1% by weight of the final C/C composite material.
Document WO 2006/067184 recommends impregnating a fiber texture of plies used for making the preform with a sol-gel type solution or a colloidal suspension so as to obtain a dispersion of grains of oxides such as the oxides of titanium (TiO2), of zirconium (ZrO2), of hafnium (HfO2), and of silicon (SiO2). Subsequent heat treatment transforms those oxide grains into carbide grains.
Document EP 1 748 036 describes impregnating a carbon fiber substrate with a slip containing a carbon-precursor resin and grains of metallic oxide, e.g. SiO2, TiO2, ZrO2, . . . . After heat treatment, a C/C composite material is obtained containing carbide grains obtained by transforming oxide particles. The examples indicate the use of oxide grains having a size of several micrometers.
Document EP 0 507 564 describes making a C/C composite material part by mixing carbon fibers, ceramic powder, and carbon powder, molding, and sintering, the ceramic powder being for example an oxide such as SiO2, TiO2, ZrO2, . . . , or a nitride. The use of ZrO2 powder made of micrometer-sized grains is mentioned in Example 2, with the quantity of ZrO2 in the final composite material being 6.2%. It should be observed that amongst the ceramic powders envisaged, ZrO2 is far from giving the best wear results.
Document EP 0 404 571 describes a method similar to that of EP 0 507 564, but for forming a sliding part having a low coefficient of friction.